Biomedical subjects

M Desmurget

Publications and source records attributed to M Desmurget.

At least 19 recordsLinked to original sources

PubMed2001

Functional anatomy of nonvisual feedback loops during reaching: a positron emission tomography study.

Reaching movements performed without vision of the moving limb are continuously monitored, during their execution, by feedback loops (designated nonvisual). In this study, we investigated the functional anatomy of these nonvisual loops using positron emission tomography (PET). Seven subjects had to "look at" (eye) or "look and point to" (eye-arm) visual targets whose location either remained stationary or changed undetectably during the ocular saccade (when vision is suppressed). Slightly changing the target location during gaze shift causes an increase in the amount of correction to be generated. Functional anatomy of nonvisual feedback loops was identified by comparing the reaching condition involving large corrections (jump) with the reaching condition involving small corrections (stationary), after subtracting the activations associated with saccadic movements and hand movement planning [(eye-arm-jumping minus eye-jumping) minus (eye-arm-stationary minus eye-stationary)]. Behavioral data confirmed that the subjects were both accurate at reaching to the stationary targets and able to update their movement smoothly and early in response to the target jump. PET difference images showed that these corrections were mediated by a restricted network involving the left posterior parietal cortex, the right anterior intermediate cerebellum, and the left primary motor cortex. These results are consistent with our knowledge of the functional properties of these areas and more generally with models emphasizing parietal-cerebellar circuits for processing a dynamic motor error signal.

Adult↗

PubMed2000

Forward modeling allows feedback control for fast reaching movements.

Delays in sensorimotor loops have led to the proposal that reaching movements are primarily under pre-programmed control and that sensory feedback loops exert an influence only at the very end of a trajectory. The present review challenges this view. Although behavioral data suggest that a motor plan is assembled prior to the onset of movement, more recent studies have indicated that this initial plan does not unfold unaltered, but is updated continuously by internal feedback loops. These loops rely on a forward model that integrates the sensory inflow and motor outflow to evaluate the consequence of the motor commands sent to a limb, such as the arm. In such a model, the probable position and velocity of an effector can be estimated with negligible delays and even predicted in advance, thus making feedback strategies possible for fast reaching movements. The parietal lobe and cerebellum appear to play a crucial role in this process. The ability of the motor system to estimate the future state of the limb might be an evolutionary substrate for mental operations that require an estimate of sequelae in the immediate future.

Journal Article↗

PubMed2000

Functional adaptation of reactive saccades in humans: a PET study.

It is known that the saccadic system shows adaptive changes when the command sent to the extraocular muscles is inappropriate. Despite an abundance of supportive psychophysical investigations, the neurophysiological substrate of this process is still debated. The present study addresses this issue using H2(15)O positron emission tomography (PET). We contrasted three conditions in which healthy human subjects were required to perform saccadic eye movements toward peripheral visual targets. Two conditions involved a modification of the target location during the course of the initial saccade, when there is suppression of visual perception. In the RAND condition, intra-saccadic target displacement was random from trial-to-trial, precluding any systematic modification of the primary saccade amplitude. In the ADAPT condition, intra-saccadic target displacement was uniform, causing adaptive modification of the primary saccade amplitude. In the third condition (stationary, STAT), the target remained at the same location during the entire trial. Difference images reflecting regional cerebral-blood-flow changes attributable to the process of saccadic adaptation (ADAPT minus RAND; ADAPT minus STAT) showed a selective activation in the oculomotor cerebellar vermis (OCV; lobules VI and VII). This finding is consistent with neurophysiological studies in monkeys. Additional analyses indicated that the cerebellar activation was not related to kinematic factors, and that the absence of significant activation within the frontal eye fields (FEF) or the superior colliculus (SC) did not represent a false negative inference. Besides the contribution of the OCV to saccadic adaptation, we also observed, in the RAND condition, that the saccade amplitude was significantly larger when the previous trial involved a forward jump than when the previous trial involved a backward jump. This observation indicates that saccade accuracy is constantly monitored on a trial-to-trial basis. Behavioral measurements and PET observations (RAND minus STAT) suggest that this single-trial control of saccade amplitude may be functionally distinct from the process of saccadic adaptation.

Adaptation, Physiological↗

PubMed2000

Postural invariance in three-dimensional reaching and grasping movements.

The question of whether the final arm posture to be reached is determined in advance during prehension movements remains widely debated. To address this issue, we designed a psychophysical experiment in which human subjects were instructed to reach and grasp, with their right arm, a small sphere presented at various locations. In some trials the sphere remained stationary, while in others (the perturbed trials) it suddenly jumped, at movement onset, to a new unpredictable position. Our data indicate that the final configuration of the upper limb is highly predictable for a given location of the sphere. For movements directed at stationary objects, the variability of the final arm posture was very small in relation to the variability allowed by joint redundancy. For movements directed at "jumping" objects, the initial motor response was quickly amended, allowing an accurate grasp. The final arm posture reached at the end of the perturbed trials was neither different from nor more variable than the final arm posture reached at the end of the corresponding stationary trials (i.e. the trials sharing the same final object location). This latter result is not trivial, considering both joint redundancy and the motor reorganization imposed by the change in sphere location. In contrast to earlier observations, our data cannot be accounted for by biomechanical or functional factors. Indeed, the spherical object used in the present study did not constrain the final arm configuration or the hand trajectory. When considered together, our data support the idea that the final posture to be reached is planned in advance and used as a control variable by the central nervous system.

Adult↗

PubMed2000

Proprioception does not quickly drift during visual occlusion.

Several perceptual studies have shown that the ability to estimate the location of the arm degrades quickly during visual occlusion. To account for this effect, it has been suggested that proprioception drifts when not continuously calibrated by vision. In the present study, we re-evaluated this hypothesis by isolating the proprioceptive component of position sense (i.e., the subjects were forced to rely exclusively on proprioception to locate their hand, which was not the case in earlier studies). Three experiments were conducted. In experiment 1, subjects were required to estimate the location of their unseen right hand, at rest, using a visual spot controlled by the left hand through a joystick. Results showed that the mean accuracy was identical whether the localization task was performed immediately after the positioning of the hand or after a 10-s delay. In experiments 2 and 3, subjects were required to point, without vision of their limb, to visual targets. These two experiments relied on the demonstration that biases in the perception of the initial hand location induced systematic variations of the movement characteristics (initial direction, final accuracy, end-point variability). For these motor tasks, the subjects did not pay attention to the initial hand location, which removed the possible occurrence of confounding cognitive strategies. Results indicated that movement characteristics were, on average, not affected when a 15-s or 20-s delay was introduced between the positioning of the arm at the starting point and the presentation of the target. When considered together, our results suggest that proprioception does not quickly drift in the absence of visual information. The potential origin of the discrepancy between our results and earlier studies is discussed.

Adult↗

PubMed2000

An 'automatic pilot' for the hand in human posterior parietal cortex: toward reinterpreting optic ataxia.

We designed a protocol distinguishing between automatic and intentional motor reactions to changes in target location triggered at movement onset. In response to target jumps, but not to a similar change cued by a color switch, normal subjects often could not avoid automatically correcting fast aiming movements. This suggests that an 'automatic pilot' relying on spatial vision drives fast corrective arm movements that can escape intentional control. In a patient with a bilateral posterior parietal cortex (PPC) lesion, motor corrections could only be slow and deliberate. We propose that 'on-line' control is the most specific function of the PPC and that optic ataxia could result from a disruption of automatic hand guidance.

Adult↗

PubMed1999

Role of the posterior parietal cortex in updating reaching movements to a visual target.

The exact role of posterior parietal cortex (PPC) in visually directed reaching is unknown. We propose that, by building an internal representation of instantaneous hand location, PPC computes a dynamic motor error used by motor centers to correct the ongoing trajectory. With unseen right hands, five subjects pointed to visual targets that either remained stationary or moved during saccadic eye movements. Transcranial magnetic stimulation (TMS) was applied over the left PPC during target presentation. Stimulation disrupted path corrections that normally occur in response to target jumps, but had no effect on those directed at stationary targets. Furthermore, left-hand movement corrections were not blocked, ruling out visual or oculomotor effects of stimulation.

Humans↗

PubMed1998

Final posture of the upper limb depends on the initial position of the hand during prehension movements.

The question of knowing how the nervous system transforms a desired position and orientation of the hand into a set of arm and forearm angles has been widely addressed during the last few decades. Despite this fact, it still remains unclear as to whether a unique posture of the arm is associated with every location and orientation of the hand in space. The main objective of the present study a was to address this question. To this end, we studied a prehension task requiring human subjects to reach and grasp a cylindrical object presented at different locations, along variable orientations. In contrast to previous investigations, we considered the influence of the initial position of the hand. Results showed that the posture of the arm: (1) varied systematically as a function of the movement starting point; (2) was stereotyped for a particular subject given a configuration of the object and a movement starting location; (3) was altered at both the distal and proximal levels when the orientation of the object was changed; (4) was similarly influenced by the experimental factors in all the subjects, except one. When considered together, the previous results support three main conclusions: First, the nervous system solves the joint redundancy problem using fixed strategies. Second, these fixed strategies do not provide a single correspondence between hand configuration and arm posture. Third, the position and orientation of the hand in space are unlikely to be controlled through separate independent neural pathways.

Adult↗

PubMed1998

From eye to hand: planning goal-directed movements.

The nature of the neural mechanisms involved in movement planning still remains widely unknown. We review in the present paper the state of our knowledge of the mechanisms whereby a visual input is transformed into a motor command. For the sake of generality, we consider the main problems that the nervous system has to solve to generate a movement, that is: target localization, definition of the initial state of the motor apparatus, and hand trajectory formation. For each of these problems three questions are addressed. First, what are the main results presented in the literature? Second, are these results compatible with each other? Third, which factors may account for the existence of incompatibilities between experimental observations or between theoritical models? This approach allows the explanation of some of the contradictions existing within the movement-generation literature. It also suggests that the search for general theories may be in vain, the central nervous system being able to use different strategies both in encoding the target location with respect to the body and in planning hand displacement. In our view, this conclusion may advance the field by both opening new lines of research and bringing some sterile controversies to an end.

Animals↗

PubMed1998

Functional anatomy of saccadic adaptation in humans.

Positron emission tomography (PET) was used to investigate the neurophysiological substrate of human saccadic adaptation. Subjects made saccadic eye movements toward a visual target that was displaced during the course of the initial saccade, a time when visual perception is suppressed. In one condition, displacement was random from trial to trial, precluding any systematic modification of the initial saccade amplitude. In the second condition, the direction and magnitude of displacement were consistent, causing adaptative modification of the initial saccade amplitude. PET difference images reflecting metabolic changes attributable to the process of saccadic adaptation showed selective activation of the medioposterior cerebellar cortex. This localization is consistent with neurophysiological findings in monkeys and brain-lesioned humans.

Adaptation, Physiological↗

PubMed1998

Pointing errors reflect biases in the perception of the initial hand position.

By comparing the visuomotor performance of 10 adult, normal subjects in three tasks, we investigated whether errors in pointing movements reflect biased estimations of the hand starting position. In a manual pointing task with no visual feedback, subjects aimed at 48 targets spaced regularly around two starting positions. Nine subjects exhibited a similar pattern of systematic errors across targets, i.e., a parallel shift of the end points that accounted, on average, for 49% of the total variability. The direction of the shift depended on the starting location. Systematic errors decreased dramatically in the second condition where subjects were allowed to see their hand before movement onset. The third task was to use a joystick held by the left hand to estimate the location of their (unseen) right hand. The systematic perceptual errors in this condition were found to be highly correlated with the motor errors in the first condition. The results support the following conclusions. 1) Kinesthetic estimation of hand position may be consistently biased. Some of the mechanisms responsible for these biases are always active, irrespective of whether position is estimated overtly (e.g., with a matching paradigm), or covertly as part of the motor planning for aimed movements. 2) Pointing errors reflect to a significant extent the erroneous estimation of initial hand position. This suggests that aimed hand movements are planned vectorially, i.e., in terms of distance and direction, rather than in terms of absolute position in space.

Adult↗

PubMed1997

Viewing the hand prior to movement improves accuracy of pointing performed toward the unseen contralateral hand.

It is now well established that the accuracy of pointing movements to visual targets is worse in the full open loop condition (FOL; the hand is never visible) than in the static closed loop condition (SCL; the hand is only visible in static position prior to movement onset). In order to account for this result, it is generally admitted that viewing the hand in static position (SCL) improves the movement planning process by allowing a better encoding of the initial state of the motor apparatus. Interestingly, this wide-spread interpretation has recently been challenged by several studies suggesting that the effect of viewing the upper limb at rest might be explained in terms of the simultaneous vision of the hand and target. This result is supported by recent studies showing that goal-directed movements involve different types of planning (egocentric versus allocentric) depending on whether the hand and target are seen simultaneously or not before movement onset. The main aim of the present study was to test whether or not the accuracy improvement observed when the hand is visible before movement onset is related, at least partially, to a better encoding of the initial state of the upper limb. To address this question, we studied experimental conditions in which subjects were instructed to point with their right index finger toward their unseen left index finger. In that situation (proprioceptive pointing), the hand and target are never visible simultaneously and an improvement of movement accuracy in SCL, with respect to FOL, may only be explained by a better encoding of the initial state of the moving limb when vision is present. The results of this experiment showed that both the systematic and the variable errors were significantly lower in the SCL than in the FOL condition. This suggests: (1) that the effect of viewing the static hand prior to motion does not only depend on the simultaneous vision of the goal and the effector during movement planning; (2) that knowledge of the initial upper limb configuration or position is necessary to accurately plan goal-directed movements; (3) that static proprioceptive receptors are partially ineffective in providing an accurate estimate of the limb posture, and/or hand location relative to the body; and (4) that static visual information significantly improves the representation provided by the static proprioceptive channel.

Fingers↗

PubMed1997

Postural control of three-dimensional prehension movements.

This experiment was carried out to test the hypothesis that three-dimensional upper limb movements could be initiated and controlled in the joint space via a mechanism comparing an estimate of the current postural state of the upper arm with a target value determined by one specific inverse static transform converting the coordinates of the object into a set of arm, forearm, and wrist angles. This hypothesis involves two main predictions: 1) despite joint redundancy, the posture reached by the upper limb should be invariant for a given context; and 2) a movement programmed in joint space should exhibit invariant characteristics of the joint covariation pattern as well as a corresponding variable hand path curvature in the task space. To test these predictions, we examined prehension movements toward a cylindrical object presented at a fixed spatial location and at various orientations without vision of the moving limb. Once presented, the object orientation was either kept constant (unperturbed trials) or suddenly modified at movement onset (perturbed trials). Three-dimensional movement trajectories were analyzed in both joint and task spaces. For the unperturbed trials, the task space analysis showed a variable hand path curvature depending on object orientation. The joint space analysis showed that the seven final angles characterizing the upper limb posture at hand-to-object contact varied monotonically with object orientation. At a dynamic level, movement onset and end were nearly identical for all joints. Moreover, for all joints having a monotonic variation, maximum velocity occurred almost simultaneously. For the elbow, the only joint presenting a reversal, the reversal was synchronized with the time to peak velocity of the other joint angles. For the perturbed trials, a smooth and complete compensation of the movement trajectory was observed in the task space. At a static level the upper limb final posture was identical to that obtained when the object was initially presented at the orientation following the perturbation. This result was particularly remarkable considering the large set of comfortable postures allowed by joint redundancy. At a dynamic level, the joints' covariation pattern was updated to reach the new target posture. The initial synergies were not disrupted by the perturbation, but smoothly modified, the different joints' movements ending nearly at the same time. Taken together, these results support the hypothesis that prehension movements are initiated and controlled in the joint space on the basis of a joint angular error vector rather than a spatial error vector.

Adult↗

PubMed1997

Constrained and unconstrained movements involve different control strategies.

This experiment was carried out to test whether or not the rules governing the execution of compliant and unconstrained movements are different (a compliant motion is defined as a motion constrained by external contact). To answer this question we examined the characteristics of visually directed movements performed with either the index fingertip (unconstrained) or a hand-held cursor (compliant). For each of these categories of movements, two experimental conditions were investigated: no instruction about hand path, and instruction to move the fingertip along a straight-line path. The results of the experiment were as follows: 1) The spatiotemporal characteristics of the compliant and unconstrained movements were fundamentally different when the subjects were not required to follow a specific hand path. 2) The instruction to perform straight movements modified the characteristics of the unconstrained movements, but not those of the compliant movements. 3) The target eccentricity influenced selectively the curvature of the "unconstrained-no path instruction" movements. Taken together, these results suggest that compliant and unconstrained movements involve different control strategies. Our data support the hypothesis that unconstrained motions are, unlike compliant motions, not programmed to follow a straight-line path in the task space. These observations provide a theoretical reference frame within which some apparently contradictory results reported in the movement generation literature may be explained.

Data Interpretation, Statistical↗

PubMed1996

Integrated control of hand transport and orientation during prehension movements.

At a descriptive level, prehension movements can be partitioned into three components ensuring, respectively, the transport of the arm to the vicinity of the target, the orientation of the hand according to object tilt, and the grasp itself. Several authors have suggested that this analytic description may be an operational principle for the organization of the motor system. This hypothesis, called "visuomotor channels hypothesis," is in particular supported by experiments showing a parallelism between the reach and grasp components of prehension movements. The purpose of the present study was to determine whether or not the generalization of the visuomotor channels hypothesis, from its initial form, restricted to the grasp and transport components, to its actual form, including the reach orientation and grasp components, may be well founded. Six subjects were required to reach and grasp cylindrical objects presented at a given location, with different orientations. During the movements, object orientation was either kept constant (unperturbed trials) or modified at movement onset (perturbed trials). Results showed that both wrist path (sequence of positions that the hand follows in space), and wrist trajectory (time sequence of the successive positions of the hand) were strongly affected by object orientation and by the occurrence of perturbations. These observations suggested strongly that arm transport and hand orientation were neither planned nor controlled independently. The significant linear regressions observed, with respect to the time, between arm displacement (integral of the magnitude of the velocity vector) and forearm rotation also supported this view. Interestingly, hand orientation was not implemented at only the distal level, demonstrating that all the redundant degrees of freedom available were used by the motor system to achieve the task. The final configuration reached by the arm was very stable for a given final orientation of the object to grasp. In particular, when object tilt was suddenly modified at movement onset, the correction brought the upper limb into the same posture as that obtained when the object was initially presented along the final orientation reached after perturbation. Taken together, the results described in the present study suggest that arm transport and hand orientation do not constitute independent visuomotor channels. They also further suggest that prehension movements are programmed, from an initial configuration, to reach smoothly a final posture that corresponds to a given "location and orientation" as a whole.

Adult↗

PubMed1995

Representation of hand position prior to movement and motor variability.

Pointing accuracy of six human subjects was measured in two blocked conditions where the hand was either never visible (T: target only) or only visible in static position prior to movement onset (H+T: hand+target). It was shown in condition H+T that, viewing the hand prior to movement greatly decreased end-point variability compared with condition T. This effect was associated with a significant modification of the movement kinematics: the H+T condition induced a shortened acceleration phase with a corresponding lengthened deceleration phase, compared with the T condition. These results led us to the hypothesis that viewing the hand prior to movement onset allowed a decrease of pointing variability through a feedback process. This hypothesis was further tested by turning the target off during the deceleration phase of the movement at half peak velocity. It was shown that turning the target off had no effect upon the T condition but induced a significant increase of pointing variability in the H+T condition. This result suggests that vision of the static hand enhances the proprioceptive localization of the limb and allows for a better visual to kinesthesic feedback.

Adult↗

PubMed1995

Vectorial coding of movement: vision, proprioception, or both?

1. Subjects were asked to point toward visual targets without visual reafference from the moving hand in two conditions. In both conditions the pointing fingertip was viewed only before movement onset. 2. In one condition, the pointing fingertip was viewed through prisms that created a visual displacement without altering the view of the target. In another experimental condition, vision of the fingertip was not displaced. Comparison of these two conditions showed that virtually shifting finger position before movement through prisms induced a pointing bias in the direction opposite to the shift. The extent of this pointing bias was about one third of the prismatic shift applied to the fingertip. 3. Analysis of movement initial direction demonstrated that it was also less deviated than predicted from the prismatic shift. In addition, the reaction time and movement time of the reaching movement were increased. 4. This result is interpreted in the framework of the vectorial coding of reaching movement. Proprioception and vision provide two possible sources of information about initial hand position, i.e., the origin of the movement vector. The question remains as to how these two sources of information interact in specifying initial hand position when they are simultaneously available. 5. Our results are thus discussed with respect to a visual-to-visual movement vector hypothesis and a proprioceptive-to-visual vector hypothesis. It is argued that the origin of the putative movement vector is encoded by weighted fusion of the visual and the proprioceptive information about hand initial position.

Adult↗

PubMed1995

Postural and synergic control for three-dimensional movements of reaching and grasping.

1. A fundamental question about motor control is related to the nature of the representations used by the nervous system to program the movement. Theoretically, arm displacement can be encoded either in task (extrinsic) or in joint (intrinsic) space. 2. The present study investigated the organization of complex movements consisting of reaching and grasping a cylindrical object presented along different orientations in space. In some trials, object orientation was suddenly modified at movement onset. 3. At a static level, the final limb angles were highly predictable despite the wide range of possible postures allowed by articular redundancy. Moreover, when object orientation was unexpectedly modified at movement onset, the final angular configuration of the limb was identical to that obtained when the object was initially presented along the orientation reached after the perturbation. 4. At a dynamical level, a generalized synergy was observed, and tight correlations were noted between all joint angles implicated in the movement with the exception of elbow flexion. For this joint angle, which did not vary monotonically, strong partial correlations were however observed before and after movement reversal. 5. These results suggest that natural movements are mostly carried out in joint space by postural transitions.

Arm↗